Printers, copiers, fax machines, scanners and the like use a variety of different pick mechanisms to deliver individual sheets of media to a delivery or indexing system. It is known to provide a stack of media in a tray, which may be vertical, horizontal or at some angle therebetween, and to feed individual sheets from the stack to the device. For trouble free operation of the device, it is necessary that only a single sheet be fed at a time. Thus, the pick mechanism and sheet feed structure must include some means for individualizing or separating a single sheet from the stack of sheets. In very high-speed devices that are often costly, it is known to use sheet feeders having relatively complex mechanical mechanisms for separating the individual sheets. However, there is a need also for lower cost printers, copiers, fax machines and the like. These machines must also separate and feed media reliably, but with less expensive and less complicated mechanisms. Therefore, simple, reliable, inexpensive sheet separating devices are required.
Passive separation devices are known to have a slanted rubber element against which an advancing piece of media is driven. The media is driven from the top of a stack of media by a spring-biased feeding wheel. Separation occurs from the friction of the advancing media edge against the rubber surface. The angle of the rubber surface in relation to the advancing media and the frictional coefficient of the rubber impact the performance of such devices. If the media stack is large, the stack covers much of the rubber surface, and the exposed distance over which separation can occur is small. Further, the feed wheel rides on top of the media stack, biased thereagainst by a spring. When the media stack is tall, the spring force exerted against the stack through the drive wheel is large and friction between sheets is high. Coupled with the short separation distance available, these conditions can lead to double feeding. Conversely, when the media stack is short, the spring force against the stack is lower, and may be inadequate to empty the media tray if the spring is adjusted to eliminate potential double feeding from a tall media stack. Further problems are experienced if the stack of media is positioned too closely to the rubber surface. Double feeding and jamming can result if the stack of media is forced against the rubber surface.
Designing a simple but effective separating device is complicated by several factors. As mentioned previously, sheet feed devices can be horizontal, vertical or at some angle between horizontal and vertical. Further, it is common and desirable for the devices such as printers, copiers and fax machines to process a variety of different materials. For example, a printer may routinely handle relatively lightweight, draft grade papers, heavier weight bond papers, card stock, transparencies and envelopes. To simplify the device and reduce cost, while also reducing the chance for mechanical failure, it is desirable that many functions of the machine be relatively passive, that is, the function performed with minimal mechanical movement or operation.
While known separating devices have functioned satisfactorily to some extent, as speeds increase more definite reliable separation is required. Further, it is desirable that the separating device function consistently with different types of media, from the top piece of media in a stack to the bottom piece of media in the stack.